1. Field of the Invention
This invention relates to target assemblies for electronic theft detection systems and more particularly it concerns novel wafers which contain electronic target circuits and which can be temporarily attached to articles of merchandise.
2. Description of the Prior Art
In U.S. Pat. No. 3,500,373 there is described an electronic theft detection system which may be used in a retail store to protect articles of merchandise from theft. As described in that patent, a wafer is provided for each article to be protected. The wafer is a generally flat, plastic element which contains a frequency selective electronic circuit and which can be temporarily, but securely, attached to the article. If any article of merchandise, having such a wafer attached to it, is carried through a checkpoint or interrogation zone, such as at or near the store exit, the electronic circuit in the wafer will react with an electromagnetic interrogation field in the checkpoint zone and will cause an alarm to be sounded.
The wafers of the prior art have been of laminate construction, comprising upper and lower flat plastic elements with recesses in their mutually facing surfaces. A prewound coil of insulated electrical wire is provided with a capacitor connected across its ends to form a resonant electrical circuit; and the coil-capacitor assembly is laid in the recesses of one of the plastic elements before the elements are superimposed on each other and heat welded together. One of the plastic elements is also provided with a housing in which a tack shank gripping mechanism, or lock, is fitted. The other element is provided with the gripping mechanism so that the shank of a tack-like fastener can be inserted into the gripping mechanism.
In some prior art wafer constructions the tack-like fastener is secured by means of a "Tinnerman" type element to one end of an arm which is hinged at its other end to the edge of the wafer. The head of the fastener is countersunk into a recess formed in the arm and mutually facing abutments are provided on the arm and on the wafer surrounding the tack shank. These prior art wafer constructions, however, are susceptible to tampering and unauthorized removal because it is sometimes possible, by pulling back slightly on the fastener or on the material to which it is attached, to expose enough of its shank so that a severing tool can be inserted and applied to the shank.